This invention relates to novel methods for preparing high purity metal oxides, and particularly to high purity alkaline earth metal oxides such as strontium oxide (SrO). SrO is useful, for example, as a raw material in the production of high temperature superconductors.
Strontium oxide is frequently used in the preparation of certain types of high temperature superconducting materials, particular high temperature superconducting ceramics. High temperature superconductors are materials that conduct electricity essentially perfectly at temperatures substantially above absolute zero. For example, normal superconductors normally operate at a temperatures that are only slightly (e.g., three or four degrees above absolute zero). Many preferred high-temperature superconductors operate at substantially higher temperatures. For example, one copper oxide-based high-temperature superconductor is composed of bismuth, strontium, calcium, copper and oxygen (frequently referred to as BSCCO or BIS-ko), and this material has a critical temperature of about 100° K. Such BSCCO superconductor are typically made up of a repeating series of layers: two bismuth oxide layers, a strontium oxide layer, and two copper oxide layers with some calcium atoms sandwiched between them. Such copper oxide superconductors may be used, for example, in electric power transformers and in mobile-phone base stations. Currently such materials are being tested in experimental biomedical devices, such as magnetic resonance imaging machines.
High temperature superconductors have also been developed from systems comprising [Y or La (lanthanoids)]-(Ba or Sr)—Cu—O (with a critical temperature of about 90° K).
One difficulty with the use of such materials is that relatively small amounts of impurities in one or more of the layers may interfere with the mechanism creating the superconductivity. The use of strontium oxide (SrO) for applications such as this thus generally requires a material of relatively high purity, particularly with respect to metal impurities, in which case even trace amounts of such impurities may be detrimental to the functioning of such materials. Furthermore, such materials frequently are preferred to have a relatively fine and homogeneous particle size distribution in order to enhance the use in such applications. In addition, it is desirable in many such applications for the assay of the material to be relatively high, which is generally difficult to achieve due the hygroscopic properties of SrO and the high tendency to absorb CO2 from the atmosphere.
One known method for the preparation of SrO is thermal decomposition of strontium carbonate (SrCO3) or a strontium hydroixide such as strontium dihydroxide (Sr(OH)2). For example, WO 97119894 describes the preparation of SrO by thermal treatment of Sr(OH)2 at a temperature between 400° C. and 900° C. One disadvantage of this method is that Sr(OH)2 has a melting point of 375° C., and as a result this process proceeds in a highly alkaline melt which has a tendency to attack or otherwise degrade the material from with the reaction vessels are made, such as ceramic and metal. It is very disadvantageous, and perhaps not practically possible under such conditions to produce high purity SrO.
Preparation of SrO by thermal treatment of SrCO3 has been disclosed. Generally, the decomposition step that has been heretofore commonly used involves the introduction of solid SrCO3 particles into a reaction vessel heated to temperatures of about 1290° C. under atmospheric pressure. One disadvantage of such method is that the SrO prepared in this manner tends to form extremely hardened solid materials, and removal of the reaction product from the crucible or other vessel can be difficult and/or inefficient and/or ineffective.
One proposed alternative is suggested in U.S. Pat. Nos. 1,782,830 and 2,382,909. The process disclosed in these patents mix the SrCO3 with carbon black before the thermal treatment process. One disadvantage of this method is that non-volatile or combustible impurities create impurities in the SrO. A similar process using a rotary furnace is disclosed in DT 24 19 822.
Attempts have been made to overcome the hardening issue by performing the thermal treatment step under a hydrogen atmosphere (U.S. Pat. No. 1,947,952) or in vacuum (U.S. Pat. No. 1,729,428). Both methods require advanced, relatively expensive furnace configurations, and associated high costs in investment and potentially operation to carry out such preparation procedures on an industrial scale.
U.S. Pat. No. 3,743,691 suggest purifying commercially available SrCO3 by first thermally treating relatively low purity SrCO3 using a rotary furnace or a fluid bed furnace to form a mixture containing SrO. This step is then followed by hydrating the SrO in the reaction product to produce Sr(OH)2, followed by dissolution of the dihydroxide in water. Insoluble solids contained in the reaction product are then removed by filtration. The filtrate is then exposed to a carbonation step in which inorganic carbonates are added to the aqueous solution and SrCO3 is formed. This process has disadvantages. For example, the SrO which is formed as an intermediate in this process contains substantial impurities. Also, another disadvantage is that the yield of purified SrCO3 per unit volume is relatively low due to the low solubility of Sr(OH)2 in water.